1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine to determine the concentration of O2, an air-fuel ratio, or the concentration of NOx in exhaust emissions, and more particularly to an improved structure of such a type of gas sensor equipped with gas inlets designed to create a desired flow of gas to be measured and minimize the concentration of thermal stress around the gas inlets.
2. Background Art
Conventionally, gas sensors are used for burning control of internal combustion engines for automotive vehicles. As a typical example, a gas sensor is installed in an exhaust pipe of an automotive engine to measure the concentration of a specified gas contained in exhaust emission of the engine. A gas sensor of this type consists essentially of a gas sensor element disposed within a hollow cylindrical housing, an air cover installed on a base portion of the housing, and a protective gas cover installed on a tip portion of the housing. The gas cover has formed therein a plurality of gas inlet holes through which a gas to be measured is admitted.
For example, U.S. Pat. No. 6,346,179 B1 (Japanese Patent First Publication No. 2001-99807) discloses a unique structure of a protective gas cover as used in a gas sensor.
FIGS. 9 and 10 show an example of a protective gas cover of the type as described above.
The protective gas cover 91 is usually required to be designed to minimize a direct hit of a flow of gas to be measured on the sensor element exposed inside the protective gas cover.
In the case where it is only required to draw a flow of gas into the protective gas cover 91, the protective gas cover 91 may have gas inlet holes 910 formed by merely punching out a side wall thereof. Although not illustrated, additional gas inlets may be formed in a straight section of the side wall beneath the gas inlet holes 910.
Usually, exhaust gases emitted from automotive engines contain various harmful substances which result in accelerated deterioration of the sensor element 2. The structure of the gas inlet holes 910 results in a flow of the gas which is, as indicated by an arrow m in FIG. 9, oriented to cause a direct hit on an outer surface of the sensor element 2, so that the sensor element 2 is exposed to the harmful substances, thus leading to the accelerated deterioration of the sensor element 2.
To encourage an approach to solution of the above drawback, there has been proposed a protective gas cover, as demonstrated in FIGS. 11 and 12.
The protective cover 92 has gas inlet holes 920 each equipped with a louver which is designed to avoid a direct hit of the gas on the sensor element 2. Each of the louvers is formed by making a U-shaped cut 922 in the side wall of the gas cover 92 to form a tab 970 and bending the tab 970 inwardly of the gas cover 92 at a base 923 of the tab 970. The louvers, as illustrated in FIG. 11, serve to create flows of the gas oriented upward, as indicated by arrows n, thereby avoiding a direct hit of the gas on the sensor element 2.
The louvers of the illustrated type are, however, lower in mechanical strength at the base 923. When exposed to hot exhaust gas of the engine, the base 923 undergoes a great degree of thermal stress, which may, as illustrated in FIG. 12, result in cracks 921 extending from the cut 922.